In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, handover is a vital part of any cellular communications network. A handover may be defined as the process of transferring an ongoing connection of a wireless device from one radio access network node (denoted the serving radio access network node) to another radio access network node (denoted the target radio access network node) in order to accomplish a transparent service over a large coverage area. The handover should be performed without any loss of data transmission to/from the wireless device and with as small interrupt as possible for the wireless device.
To enable a handover, it is necessary to find a suitable target cell as served by the target radio access network node, and to ensure that it is possible to sustain reliable communication to/from the wireless device in the target cell. Candidates for suitable target radio access network nodes (and/or target cells) are usually stored in so-called neighbor lists, which are stored at least at the serving radio access network node. To make sure that it is possible to sustain reliable communication to/from the wireless device in the target cell, the connection quality in the target cell needs to be estimated before the handover can be performed.
The connection quality of the target cell is commonly estimated by measurements related to the wireless device. Downlink (DL, i.e., transmission from radio access network node to wireless device) and/or uplink (UL, i.e., transmission to radio access network node from wireless device) measurements may be considered. Relying solely on uplink measurements may be unreliable, since the uplink connection quality can be different from the corresponding downlink connection quality. Therefore, handovers in cellular communications networks are commonly based on downlink measurements.
In existing cellular communications networks, all radio access network nodes (RANNs) continuously transmit pilot signals that wireless devices (WDs) in neighbor cells use to estimate the target cell quality. This is true in the Global System for Mobile Communications (GSM) where such pilot signals are transmitted on the broadcast control channel (BCCH), in the Universal Mobile Telecommunications System (UMTS) where such pilot signals are transmitted on the Common Pilot Channel (CPICH) and in the Long Term Evolution (LTE) telecommunications system where such pilot signals are transmitted as cell specific reference signals, as well as in WiFi where such pilot signals are transmitted as beacons. This makes it possible to estimate the quality of neighbor cells with relatively good accuracy. The WDs perform measurements periodically and report the measurements to the network (i.e., the RANN). If it is detected that the serving cell quality is getting near the candidate cell power, a more detailed measurement process or a handover procedure may be initiated.
Future cellular communications networks may use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. When the antenna is used to increase coverage, handover between narrow transmission beams in neighboring RANNs may become a necessity. The serving RANN also needs to decide if a beam switch or beam update is necessary within the own cell. The transmission beam through which the RANN is currently communicating with the WD is called the serving beam and the transmission beam it will hand over to, or switch to, is called the target beam. The serving beam and the target beam may be transmission beams of the same or different RANN.
Applying the principle of continuous transmission of pilot signals in all individual transmission beams in such a future cellular communications network may be convenient for WD measurements, but it may degrade the performance of the network. For example, continuous transmission of pilot signals in all individual transmission beams may generate a lot of interference in neighbor cells, it may consume network capacity otherwise available for data, and may lead to higher power consumption of the RANNs, since long discontinuous transmission (DTX) periods become impossible.
Further, in a communications network relying on advanced antennas with narrow transmission beams to improve coverage, it is inefficient, or sometimes even impossible, to transmit the pilot signal in all transmission beams at the same time. The natural alternative to transmit consecutively in different beams leads to longer measurement periods, slower handovers and beam updates.
Hence, there is a need for an improved triggering of mobility measurements for wireless devices.